Integrated circuits are mass produced by fabricating hundreds of identical circuit patterns on a single semiconductor wafer. One of the many different processes repeated over and over in manufacturing these integrated circuits is that of using a mask and etchant for forming a particular feature. In such a mask and etching process, a photo mask containing the pattern of the structure to be fabricated is created, then, after formation of a material layer within which the feature is to be formed, the material layer is coated with a light-sensitive material called photoresist or resist. The resist-coated material layer is then exposed to ultraviolet light through the mask, thereby transferring the pattern from the mask to the resist. The wafer is then etched to remove the material layer unprotected by the resist, and then the remaining resist is stripped. This masking process permits specific areas of the material layer to be formed to meet the desired device design requirements.
In the etching process described above, it is important that the etching selectively remove the unwanted material and that the material underlying the material layer is not excessively damaged. A common way to accomplish this is to deposit or otherwise form an etch stop layer on the wafer prior to formation of the material layer. Such etch stop layers are commonly made of a material that is resistant to the particular etching process used.
In the integrated circuit fabrication art, the property of being resistant to an etching process is called the “selectivity” of a material. The selectivity of a particular material in a particular etching process is usually defined as the etching rate of the material to be removed divided by the etching rate of the particular material. Thus, a material that is highly resistant to an etch is said to have a high selectivity.
One of the most effective, and thus common etch stop layers currently used in the fabrication of integrated circuits is silicon nitride (SiN). Unfortunately, using SiN as the etch stop layers introduces unwanted hydrogen into the features of the integrated circuits. Hydrogen is unwanted for a number of reasons. First, the unwanted hydrogen has a negative impact on the transistors, often affecting their gate oxide integrity (GOI) value, antenna gate leakage value, threshold voltage drift value, negative bias temperature instability (NBTI) value, etc. Second, when used in conjunction with ferroelectric memory cells, the unwanted hydrogen introduced by the SiN is often catastrophic. For this reason, a barrier layer is typically introduced between the ferroelectric memory cell and the SiN etch stop. Regrettably, the barrier layer provides an additional step to the already complicated manufacturing process.
Accordingly, what is needed in the art is an etch stop that does not experience, or in another aspect introduce, the problems that arise with the use of the prior art etch stops.